1. Field of the Invention
This invention relates to a process for the preparation of polyesters, in particular, to a process for the preparation of polyalkylene terephthalates having good qualities and excellent melt-forming properties by the ester-interchange reacting method.
2. Description of the Prior Art
The process for the preparation of polyesters, e.g., polyethylene terephthalate, by the ester-interchange reaction method consists of heating a mixture of a di-lower alkyl (1-4 carbon atoms) ester of terephthalic acid and ethylene glycol at a temperature approximately between 130 and 250.degree. C. for about 2 to 5 hours at normal atmospheric pressure while removing the lower alcohol formed to complete the ester-interchange reaction, and thereafter heating the obtained di-ethylene glycol ester of terephthalic acid and/or a low molecular weight condensate thereof while removing ethylene glycol therefrom at a temperature between the melting point thereof and about 300.degree. C. for about 2 to 5 hours at a reduced pressure of about 30 to 0.1 mm Hg to form polyethylene terephthalate.
The di-lower alkyl (1-4 carbon atoms) ester of terephthalic acid used in the aforesaid ester-interchange reaction method is industrially prepared from p-xylene by a method comprising the oxidation process of p-xylene. The SD process and the Witten process are the most widely employed methods. According to the SD process, p-xylene is oxidized with a molecular oxygen-containing gas, in the presence of a heavy metal catalyst and a bromine compound as the promotor, in a lower aliphatic acid solvent such as acetic acid, to form terephthalic acid, and the terephthalic acid thus formed is further esterified with a lower alcohol having from 1 to 4 carbon atoms to produce the di-lower alkyl ester of terephthalic acid. (See, for example, U.S. Pat. No. 2,833,816, Saffer et al., issued May 6, 1958.)
According to the Witten process, p-xylene is oxidized with a molecular oxygen-containing gas in the liquid phase in the presence of a heavy metal catalyst to form p-toluic acid, which is then esterified with a lower alcohol having from 1 to 4 carbon atoms to form a lower alkyl p-toluate, the lower alkyl p-toluate is again liquid phase-oxidized with a molecular oxygen-containing gas in the presence of a heavy metal catalyst to be converted to monoalkyl terephthalate, and the monoalkyl terephthalate is esterified with the lower alcohol to provide the di-lower alkyl ester of terephthalic acid. (See, for example, British patent specification No. 727,989, published Apr. 13, 1955, British patent specification No. 809,730, published Mar. 4, 1959.)
The di-lower alkyl ester of terephthalic acid obtained from p-xylene usually contains some impurities such as lower alkyl (1-4 carbon atoms) p-formylbenzoates, mono- or di-lower alkyl (1-4 carbon atoms) isophthalates, mono- or di-lower alkyl (1-4 carbon atoms) phthalates, p-toluic acid and mono-lower alkyl (1-4 carbon atoms) terephthalates in an amount of, e.g., more than about 1,000 ppm. (This di-lower alkyl ester of terephthalic acid is hereinafter referred to as the crude DAT.)
The aforesaid impurities contained in the crude DAT result in the formation of polyalkylene terephthalates having an unfavorable yellow color and lower the softening point of polyalkylene terephthalates. The crude DAT is, therefore, highly purified by a distillation and/or recrystallization processes to obtain the di-lower alkyl ester of terephthalic acid having, e.g., less than about 100 ppm of the total amounts of the impurities. (This di-lower alkyl ester of terephthalic acid is hereinafter referred to as the fiber grade DAT.)
The distillation and/or recrystallization processes, being complex ones as disclosed in, for example, Hydrocarbon Processing Vol. 54, No. 11 (November 1975), page 131-133, published by the Gulf Publishing Company, inevitably lead to high costs in the preparation of polyalkylene terephthalates. Therefore, it has strongly been desired in the synthetic fiber industry to complete a process for the preparation of polyesters having good qualities by using the di-lower alkyl ester of terephthalic acid containing more impurities than those contained in the fiber grade DAT.
The polyesters obtained by the ester-interchange reaction method have another disadvantage of poor melt-forming properties. In carrying out the ester-interchange reaction and the polycondensation reaction of the ester-interchange products, some metal compounds are used as catalysts to accelerate the reactions. These metal compounds remain in the obtained polyesters as insoluble and infusible foreign matters most of which have a diameter of more than about 20.mu., and cause some troubles in the melt-forming process of the polyesters. In manufacturing polyalkylene terephthalate filaments, for example, the foreign matters build up around the spinning orifices of a spinneret pack when a molten polyalkylene terephthalate is melt-spun through the spinneret pack, and thus make it difficult to perform the smooth melt-spinning operation because of the resulting abrupt rise of the pressure in the spinneret pack and the breakage of spun filaments. Therefore, it has also been desired in the synthetic fiber industry to provide polyesters having good melt-forming properties.
Japanese Patent Application Publication No. 33280/73, Itabashi et al., published Oct. 12, 1973 discloses a process for the preparation of polyalkylene terephthalates having a good color which comprises reacting a di-lower alkyl ester of terephthalic acid with an alkylene glycol in the presence of an aromatic aldehyde-acid, such as 2-, 3- or 4-carboxybenzaldehyde, or an alkyl ester thereof and a cobalt compound, such as cobalt acetate. Since the alkyl esters of carboxybenzaldehydes (alkyl formylbenzoates) are contained as impurities in the crude DAT, this process is preferably used for preparing the polyalkylene terephthalates from the crude DAT. This process, however, does not overcome the disadvantage caused by foreign matters in the polyalkylene terephthalates.